If "physically correct" is what you need then OpenGL probably isn't what you want. In general, OpenGL is suitable for rendering something that looks pretty good, FAST.
I don't know of any way to get fragment coverage in a shader. However, even if you could that wouldn't be adequate: Imagine you have a white (255) square on a black (0) background. You size the square so that it precisely covers one pixel. When you render it, the pixel is coloured white (255). Now imagine you divide that white square in half. You would expect that rendering the two halves would also get you a value of 255, but... When you render the first half, the pixel is 255 * 0.5 + 0 * (1 - 0.5) = 127. When you render the second half the pixel is 255 * 0.5 + 127 * (1 - 0.5) = 191. NOT 255! Therefore, multiplying alpha by coverage wouldn't work unless you're only dealing with separate polygons, which isn't the case most of the time. HTH
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